U.S. patent number 4,887,032 [Application Number 07/188,883] was granted by the patent office on 1989-12-12 for resonant vibrating structure with electrically driven wire coil and vibration sensor.
This patent grant is currently assigned to Ford Motor Company. Invention is credited to Robert E. Hetrick.
United States Patent |
4,887,032 |
Hetrick |
December 12, 1989 |
Resonant vibrating structure with electrically driven wire coil and
vibration sensor
Abstract
A vibration type sensor can make a noncontacting measurement of
position or sense the passage of an object past a point. The sensor
has a coil of wire placed on a vibrating structure. A power supply
causes current to be passed through the coil at the resonant
frequency of the vibrating structure. As an object with an attached
magnet approaches the coil, a force is exerted on the current
carrying coil thereby exciting the structure at its resonant
vibrational frequency. A piezoelectric bimorph is attached to the
structure so that it flexes with the structure producing an emf
output proportional to the vibrational amplitude. As the object
moves, the field strength at the coil, the amplitude of the
vibration and the induced emf change. The latter quantity is used
to sense the motion of the object. Alternatively, the magnet
remains fixed with respect to the coil while a material of high
magnetic permeability approaches the coil and magnet combination
thereby perturbing the magnetic field and changing the induced emf.
This method can be used to obtain a linear variation of sensor
output with position or to enable the use of the device as a timing
sensor.
Inventors: |
Hetrick; Robert E. (Dearborn
Heights, MI) |
Assignee: |
Ford Motor Company (Dearborn,
MI)
|
Family
ID: |
22694962 |
Appl.
No.: |
07/188,883 |
Filed: |
May 2, 1988 |
Current U.S.
Class: |
324/207.16;
324/260; 324/256 |
Current CPC
Class: |
G01B
7/023 (20130101); G01B 7/14 (20130101) |
Current International
Class: |
G01B
7/02 (20060101); G01B 7/14 (20060101); G01B
007/14 (); G01R 033/02 (); G01R 033/00 () |
Field of
Search: |
;324/207,208,256,257,260,261,262 ;310/318,328,330 ;73/649,654 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Snow; Walter E.
Attorney, Agent or Firm: Abolins; Peter Zerschling; Keith
L.
Claims
I claim:
1. A resonance vibrating structure including:
an adjacent external magnet;
wire coil means coupled to said vibrating structure;
electrical power supply means coupled to said coil means to pass
electric current through said coil means at the resonance
vibrational frequency of said structure;
said wire coil means being positioned on said structure so that
when current having the resonance vibrational frequency is passed
through said coil means in the presence of said external magnet, an
electromagnetic force is exerted on said structure which causes
said structure to vibrate at its resonance frequency;
a vibration sensing means positioned on the structure so that the
vibration of said structure causes an electrically measurable
output from said vibration sensing means proportional to the
vibrational amplitude;
said coil means being appropriately sized and positioned with
respect to said magnet so that the vibration induced output from
said vibration sensing means varies in a predetermined desired
manner as the magnetic field is perturbed so that the output can
serve as a sensor of relative position between an object perturbing
the magnetic field and said coil means;
feedback means coupled to said electrical power supply means and to
said vibration sensing means for feedback control, said feedback
means providing an output which controls the current through said
coil means for exciting the vibration and keeping said resonance
vibrating structure vibrating at its resonance frequency even
though the resonance frequency may be changing with external
ambient conditions such as temperature.
2. A resonance vibrating structure as recited in claim 1, further
including:
a shield of high magnetic permeability attached to the object whose
position is to be sensed, said shield being appropriately shaped
and placed so that as the object moves, said shield distorts the
magnetic field near said coil means and causes the amplitude of
vibration and, correspondingly, the output of said vibration
sensing means to vary in a desired manner, such as linearly, with
the position of the object.
3. A resonance vibrating structure as recited in claim 1, wherein
said coil means and said magnet are fixed and positioned with
respect to each other so that the passage of an object of high
magnetic permeability distorts the field at said coil means and
thus varies the vibrational amplitude and, correspondingly, the
output of said vibration sensing means thereby sensing the passage
of the object.
4. A resonance vibrating structure as recited in claim 3, wherein
said resonance vibrating structure includes:
a planar cantilever blade which vibrates in its first resonance
cantilever mode;
said vibration sensing means includes a ceramic piezoelectric
bimorph coupled to said cantilever blade which is caused to flex
when an electromagnetic force moves said blade, thereby generating
an emf which is used to sense the vibration of said blade; and
said coil means being attached to said blade in a substantially
coplanar fashion so that a portion of said coil means is located at
the base of said blade where there is reduced vibrational amplitude
while another portion of said coil means is located at the
extremity where the amplitude has its maximum value.
5. A resonance vibrating structure as recited in claim 4, wherein
said coil means is fabricated of fine wire having a plurality of
turns held together with an adhesive.
6. A resonance vibrating structure as recited in claim 4, wherein
said wire coil means is planar and formed by a deposition of metal
in combination with photolithographic techniques.
7. A resonance vibrating structure as recited in claim 6, wherein
said blade is made of silicon and at least a portion of said
feedback means is fabricated on said blade.
8. A resonance vibrating structure as recited in claim 7, further
including a planar cantilever silicon blade coupled to said coil
means, and said feedback means includes feedback control circuitry
whose input is coupled to the output of said vibration sensing
means and whose output controls the current passed through said
coil means coupled to said blade, said feedback circuitry
maintaining the current through said coil means at the resonant
frequency of said blade even though this frequency may be changing
due to external ambient conditions such as temperature.
9. A resonance vibrating structure as recited in claim 8, wherein
said vibration sensing means is a thin layer of piezoelectric
polymer attached in a coplanar fashion to said blade and provides a
sensor output for use as an input signal for feedback control to
maintain resonance conditions.
10. A resonance vibrating structure as recited in claim 8, wherein
said vibration sensing means is a silicon piezoresistive element
fabricated in said silicon blade itself and located so that
vibration of said silicon blade strains said silicon piezoresistive
element causing a change in its electrical resistance, and provides
a sensor output for use as an input signal for feedback control to
maintain resonance conditions.
11. A resonance vibrating structure includes:
an adjacent external magnet generating a magnetic field;
a wire coil means for carrying electrical current having a
plurality of turns coupled so as to vibrate as part of said
structure;
a diaphragm supporting said wire coil and driven into vibration
when an electric current is passed through the coil at the
resonance frequency of the diaphragm in the presence of a magnetic
field due to an electromagnetic force between the current and the
magnetic field and vibrating in its lowest resonance mode in which
the only node of the vibration exists at the periphery of said
diaphragm and an antinode exists near the geometrical center of
said diaphragm;
said wire coil means being attached so that a portion of each turn
exists in the regions of the antinode and node respectively;
a vibration sensing means positioned on said structure so that the
vibration of said structure causes an electrically measurable
output from said vibration sensing means proportional to the
vibrational amplitude which increases linearly with the strength of
the magnetic field at the position of the current carrying coil
near the antinode of vibration;
said wire coil means being positioned with respect to the magnet so
that the output from said vibration sensing means varies in a
predetermined manner as the magnetic field at the site of the coil
is changed due to the nearby motion of an appropriately constructed
object thereby indicating the relative position between an object
perturbing said magnetic field and said wire coil means.
12. A resonance vibrating structure as recited in claim 11, wherein
said vibration sensing means has an induced change in an electrical
parameter in response to a change in the magnitude of the magnetic
field caused by a change in the relative position of said magnet
which is attached to the object whose position relative to the coil
is being sensed.
13. A resonance vibrating structure as recited in claim 11, wherein
said vibration sensing means has an induced change in an electrical
parameter in response a change in to the magnitude of the magnetic
field caused by a change in the relative position of a separate
highly permeable magnetic material whose position relative to the
coil is to be sensed and whose motion causes changes of the
magnetic field created by said magnet, whose position relative to
the coil is fixed, said permeable magnetic material being spaced
from said magnet.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a noncontacting measurement of the
distance from an object to a reference point, and/or of the rate at
which an object passes a reference point.
2. Prior Art
Many methods are known for determining the position or distance of
an object from a reference point. Frequently, these same methods
may be used to determine whether an object is present at a position
(or a range of positions) or the temporal rate at which an object
appears at a position (e.g., as for a rotating or oscillating
object).
These methods may be divided into contacting and noncontacting
types depending on whether the object, or an extension of the
object, contacts the sensing element. A well known contacting
method involves measuring a change in electrical resistance and is
illustrated in FIG. 1. An extension 11 of an object 10 is an
electrical conductor like a metal which slides on a second
conductor 13 as the object 10 moves. The change in contact position
between the two conductors varies the length, 1, and, accordingly,
the resistance of that portion of conductor 13 appearing in an
external circuit 14. Element 11 is joined to external circuit 14 by
a flexible wire. The variation of resistance with object motion
affects the electrical characteristics of external circuit 14 in a
manner convenient for measurement, thus providing a sensing of the
position of object 10. Although useful in some applications, the
contact between element 11 and conductor 13 can be subject to wear,
vibration (leading to electrical jitter) or chemical contamination
depending on ambient conditions.
Of the numerous types of noncontacting position and/or timing
sensors, many involve electromagnetic energy in the form of
capacitive, magnetic (e.g., linear variable differential
transformer, Hall effect) or optical methods. Acoustical methods
(e.g., sonar, ultrasonic) are also widely used. Each of these
methods has its particular area of applicability arising from
considerations of cost, durability, operating environment, etc.
Also, methods using a resonantly vibrating element in combination
with electromagnetic techniques are especially advantageous for use
in the automotive environment. U.S. Pat. No. 4,297,872 to Ikeda et
al describes a vibration type transducer having a vibrator (e.g., a
hollow metal cylinder) and vibration exciters (e.g., piezoelectric
elements) which with suitable electrical activation cause the
vibrator to vibrate in one or more of its resonant modes. Vibration
detection means are located on or near the vibrator to sense the
motion and provide an electrical output. In operation, a material
or object whose property is to be sensed is suitably placed in
proximity to the vibrator so that it modifies the resonant
vibrational frequencies. For example, a fluid whose pressure is to
be sensed is introduced into the cylinder. The vibrator may be so
designed that the change in resonant frequency attending the
introduction of this fluid is proportional to the fluid pressure.
Alternatively, the temperature of the fluid may alter the resonance
in a characteristic way. Different methods may be used so that the
approach of an object ( e.g., its position) causes this resonance
frequency to change in a particular way.
Vibration detection means can be used in two ways. First, they
serve as an input to feedback electronic circuitry whose output is
applied to the vibration exciters to keep the vibration excited at
its resonant frequencies even though those frequencies may be
changing. Second, they serve as an input to additional circuitry
for processing the frequency information so that an electrical
output related to the quantity to be sensed is produced. Such a
device typifies one method of operating vibrational sensors in
which the quantity of interest modifies the vibrator's resonant
frequency. One of the disadvantages of this approach is that
sensitivity can be low because the frequency does not go to zero,
but rather returns to some fiducial value, as the perturbation or
change which causes the frequency variation is reduced to zero.
This disadvantage does not occur in the present case, and an
embodiment of this invention has high sensitivity and dynamic
range. In addition, the device is appropriate for low cost
manufacture as well as the ambient conditions peculiar to the
automotive environment.
SUMMARY OF THE INVENTION
The present invention combines the vibrational characteristics of
vibrating structures such as cantilever blades with the principles
of electromagnetism to achieve a noncontact position and/or timing
sensor. The sensor includes a coil of wire which is attached to a
vibrating cantilever blade. In the simplest case the attachment is
such that one region of the coil vibrates with large amplitude near
the unclamped extremity of the blade while another region of the
oil is stationary or vibrates with a relatively small amplitude
because it is attached near the clamped end of the blade. The coil
is largely coplanar with the blade. The free end of the blade with
attached coil can be caused to vibrate with significant amplitude
if an ac electric current of sufficient magnitude is passed through
the coil at the resonant vibrational frequency of the cantilever
blade while a magnet is positioned nearby.
An alternating force of attraction and repulsion exists between the
coil and the magnet which is proportional to the magnetic field
strength at the position of the coil and the magnitude of the ac
current in the coil. The force is transmitted to the blade from the
coil. Since the force varies at the resonant vibrational frequency
of the blade, the blade will vibrate at a relatively large
amplitude. The operation is analogous to the operation of an ac
electric motor. Motion of the blade can now be detected, for
example, if the base of the blade is attached to one end of a
length of piezoelectric bimorph whose other end is rigidly clamped.
Motion of the blade flexes the bimorph causing an ac emf to be
generated across opposing faces of the bimorph. This emf is
detected by external circuitry.
Position sensing or timing the passage of an object is effected by
varying the magnetic field at the coil in some manner. This in turn
varies the force to the blade, the amplitude of the blade vibration
and correspondingly the emf output from the piezoelectric element.
Variations in the latter quantity are detected and processed in
order to sense the motion. For example, a magnet can be attached to
the object to be sensed. The magnet is positioned so that as the
object moves, the magnet approaches or recedes from the blade
thereby varying the field strength at the coil and the output of
piezoelectric member. One advantage of this method over others
using a coil is that the signal does not depend on the time rate of
change of the magnetic field. Thus, if necessary, very slow motions
can be detected. Another potential advantage is that only very low
voltages may be required to pass current through the coil (whose
resistance can be made small) obviating the need for high-voltage
power supplies which may be required to establish the vibration in
other vibrational methods. Means other than the ceramic bimorph may
be used to sense the vibration of the blade. For example,
piezoresistors made by ion implantion near the base of a silicon
blade could be used to measure blade motion. Alternately a thin
element of polymeric piezoelectric material could be affixed near
the base of the blade to accomplish the same purpose.
In an alternate embodiment, the magnet is detached from the object
and placed in a fixed position with respect to the vibrating coil
so that a vibration is induced in the blade. A strip of magnetic
material of high permeability (e.g., iron, nickel) is attached to
the object. The coil/magnet combination is positioned so that as
the object moves the magnetic material intercepts the space between
the coil and magnet. The magnetic material distorts the magnetic
field within this space and prevents it from intercepting the coil
thereby reducing the induced vibration. By shaping and positioning
the permeable material appropriately, the change in vibrational
amplitude and emf induced in the piezoelectric member with the
position of the object can be tailored to a desired dependence. For
example, a wedge-shaped strip could lead to a desirable linear
dependence of object position with emf. Additionally, by
appropriate strip design other object motions (e.g., rotary) may be
sensed with no modification of the coil/magnet combination.
A modification of the above concept allows the coil/magnet
combination to be used as a timing sensor. A typical application of
such a sensor is to detect the passage of a tooth on a toothed
wheel in a noncontacting manner. In many applications the toothed
wheel is made of a magnetic material such as iron. The coil/magnet
combination is rigidly fixed with respect to each other so that an
ac emf is induced in the piezoelectric member. The combination is
placed close to the outer circumference of the wheel. As the tooth
of the wheel passes the combination, the magnetic field is
distorted so that more magnetic field lines pass through the tooth.
By appropriate placement of the magnet and coil, the passage of the
tooth can cause fewer field lines to intercept the coil thereby
reducing the induced piezoelectric member emf. This reduced emf can
be detected and used to sense the passage of the tooth. Of special
importance for this method, is the fact that the timing signal does
not depend on the velocity of the tooth. Further, the coil/magnet
combination can be arranged so that the tooth need not intercept
the space between the combination. This factor simplifies practical
design considerations.
If the blade is made of silicon, the coil can be fabricated with
photolithographic techniques and the signal processing electronics
integrated onto the blade itself so that the economics of silicon
batch processing can be realized.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing illustrating a contacting position
sensor which utilizes a change in electrical resistance to
determine an object's position in accordance with the prior
art.
FIG. 2 shows a vibrating cantilever structure in which a vibrating
blade is attached by a piezoelectric bimorph to a support structure
containing electrical feedthroughs in accordance with an embodiment
of this invention.
FIG. 3 shows a noncontacting position sensor adjacent to an object
to be sensed which has an attached magnet and is placed in a
particular manner so that a vibration is induced in the blade when
an ac current at the resonant frequency of the blade is passed
through the coil. The motion of the blade flexes the attached
piezoelectric member resulting in an ac emf from the piezoelectric
member.
FIG. 4 shows a planar coil of wire on the cantilever blade as well
as the induced force on the wire due to the current I in a magnetic
field B of a magnet which is attached to an object whose position
is to be sensed.
FIG. 5 illustrates the second resonance mode of a vibrating
cantilever blade with an attached coil.
FIG. 6A illustrates another embodiment of the position sensor
including a vibrating wire suspended between two posts (one of
which is a piezoelectric bimorph) and positioned properly with
respect to a magnet attached to the object whose position is to be
sensed.
FIG. 6B illustrates the first resonance vibrational mode of the
wire of FIG. 6A and the relevant parameters for computing the
applied force.
FIG. 7A illustrates another embodiment of the position sensor in
which a coil of wire vibrates on a diaphragm.
FIG. 7B illustrates the lowest frequency resonance vibrational mode
of the diaphragm of FIG. 7A appropriate for use as a position
sensor.
FIG. 8A illustrates another embodiment of the position sensor in
accordance with an embodiment of this invention for achieving a
sensor output which is linear with position in which the magnet
remains fixed and an appropriately shaped magnetic wedge is
attached to the object.
FIG. 8B is a plan view of the magnetic wedge of FIG. 8A.
FIG. 9 illustrates another embodiment in which a fixed magnet in
combination with a vibrating coil act as a timing sensor such as
for the passage, or time rate of change of passage, of the teeth on
a toothed wheel of magnetic material.
DETAILED DESCRIPTION OF THE INVENTION
A noncontacting position sensor 20 employing a piezoelectrically
sensed vibrating cantilever is shown in FIG. 2. Sensor 20 includes
a cantilever blade 21 onto which a coil (not shown in FIG. 2 for
clarity) that is largely coplanar with blade 21 has been attached.
The blade/coil combination is attached to the end of a ceramic
piezoelectric bimorph 22 which in turn is attached in a
cantilevered manner to an extension 23 of a support structure 24.
Structure 24 contains two electrical feedthroughs for the purpose
of detecting an alternating emf generated between opposing faces of
bimorph 22. One feedthrough 25 is shown with a lead wire 26
extending to the upper surface of bimorph 22. For convenience, the
extension 23 to which the opposing face of bimorph 22 is attached
can be an electrical conductor that serves as the other
feedthrough. The cantilever blade and coil combination have a
series of resonance vibrational modes at specific resonance
frequencies. The fundamental mode is one in which the free end of
blade 21 has the maximum vibrational amplitude while the only node
is at the point of support of bimorph 22. For a single blade, the
frequency of the fundamental vibrational mode is given by
wherein h is the thickness of a rectangular blade of length l from
the free end to the point of constraint, and E and .zeta. are the
Young's modulus and density, respectively. In the present case, the
cantilever is actually a composite of the bimorph extending from
its rigid support extension 23 and the attached blade/coil
combination. As a result, the resonance frequency differs from that
given above although the qualitative dependence of resonance
frequency on material parameters (e.g., length, Young's modulus,
etc.) is the same as indicated in the formula.
FIG. 3 is a side view of the invention in use as a noncontacting
position sensor. In this embodiment, an object 30, whose position
is to be sensed, has an attached permanent magnet 31. Magnet 31 is
placed on the object so that as linear motion occurs, magnet 31
approaches or recedes from the tip of a blade 32/coil 33
combination. In operation, a power supply 34 causes an alternating
current to pass through coil 33 at the fundamental vibrational
frequency of blade 32/coil 33 combination. Operation is also
possible by passing current at the frequencies of higher
vibrational modes. In the presence of the magnetic field, an
alternating force of attraction and repulsion will be exerted on
current carrying coil 33 in the same manner as for an ac motor.
Since coil 33 is rigidly attached to blade 32, the force is
transmitted to blade 32. Because the force is present at the
resonant frequency of blade 32/coil 33 combination, it vibrates at
a relatively large amplitude. This motion flexes an attached
piezoelectric member 37 which in turn generates an emf between
opposing faces of piezoelectric member 37. As the object approaches
(or recedes from) coil 33, the strength of the magnetic field
intercepting coil 33 increases (or decreases) causing the amplitude
of vibration and the induced emf to increase (or decrease). The
magnitude of the emf is detected by external circuitry 36 connected
to piezoelectric member 37 by lead wires. This emf signal serves to
sense the motion of the object.
The dimensions, materials and other design parameters for the
device are typically chosen for a specific application. As an
example, the bimorph can be made of ceramic PZT (lead zirconate
titanate) and have the approximate dimension l.sub.p =0.4 cm,
w.sub.p =0.15 cm, h.sub.p =0.05 cm. The blade can be made of
silicon of 2 mil thickness and have the dimension l.sub.b =0.4 cm,
w.sub.b =0.15 cm. A coil can be deposited on the blade with
thermally evaporated aluminum using standard photolithographic
techniques to delineate the coil pattern. As shown schematically in
FIG. 4, for example, a ten turn rectangular coil 41 of 1 micron
thickness and 25 micron width is appropriate for a silicon blade 40
described above. Current is passed through coil 41 by a power
supply 44. For such a structure, the frequency of the fundamental
cantilever resonance of blade 40 is approximately 2k Hz. The Q of
the resonance is approximately 50 and is largely determined by the
rigidness with which blade 40 is attached to a piezoelectric member
43.
Referring to FIG. 4, the force F on the blade due to the passage of
current I through coil 41 in the presence of a magnetic field B
(from magnet 42) is given by integrating the differential
expression dF=I.times.Bdl around the turns of coil 41. The vector I
stands for the magnitude of the current in the direction of the
current as defined by the shape of coil 41. Considering the
geometry of FIG. 4 and assuming that the field B is always along
the length l of blade 40, the only contributions to the force not
acting within the plane of blade 40 are those on the segments of
coil 41 along the width of blade 40. Those segments near the
attached base of blade 40 are likewise of little effect because of
the large effective stiffness of blade 40 for forces applied in
this region. Accordingly, the component of the force which is most
effective for driving blade 40 in its fundamental cantilever mode
occurs at the segments of coil 41 near the free end of blade
40.
Reasonable values for the relevant parameters are I(max)-20 mA,
B-0.05 W/m.sup.2, w.about.0.001 m, number of turns =10 which leads
to F(max) =10.sup.-4 N. For a force applied at the extremity of a
50 micron thick blade of silicon, the effective spring constant k
of the cantilever is approximately 10 N/m. At very low frequency
the maximum displacement (x.sub.o) of the blade is, x.sub.o
=F/k.about.10 micron. At resonance however, this displacement is
multiplied by the Q of the resonance which results in a maximum
displacement of the free end of the cantilever of .about.1 mm at
resonance. The magnitude of the emf generated when the vibrating
blade flexes the attached bimorph will of course depend on the
length, the effective stiffness and the piezoelectric properties of
the bimorph. Under the above conditions the maximum emf observed
was approximately 200 mV for the piezoelectric materials used in
this work. It is this electrical signal which is processed to sense
the motion of an object.
One may also exploit the second cantilever resonance illustrated in
FIG. 5. Here there are two nodes (50) of the motion, one occurring
near a piezoelectric element 53. It would be appropriate for the
segments of a coil 51 parallel to the width w to be placed at
antinodes 52 of the motion. This is because the electric current
along the width w of coil 51 will be in opposite directions at the
position of the two antinodes. Accordingly, the force on these two
segments will be in opposite directions. However, since the motion
of the blade is also opposite at these positions for this mode,
this is just the appropriate relationship between the forces that
will most effectively excite the mode. More generally, for a given
magnetic field pattern, the shape of the coil may be modified to
most effectively excite a desired mode.
This approach is not limited to the vibration of cantilever blades.
Indeed, other vibrating structures could be advantageous from the
viewpoint of manufacture or function. As an example, consider the
single wire 60 attached to two posts extending from a support
structure 64 as in FIG. 6A. Post 61 is a piezoelectric bimorph.
Electrical contacts to opposing faces of the bimorph post 61 have
been made but are not shown. Similarly, an unshown power supply
passes an ac current through the wire 60 at the frequency of the
first standing wave resonance mode of wire 60.
That frequency depends on physical parameters of wire 60 as well as
the tension existing in wire 60 caused by its attachment. If a
magnetic field originating from a magnet 62 attached to an object
63 is present in a direction perpendicular to the direction of
displacement of wire 60, a force on wire 60 will be induced in a
direction perpendicular to both the current and field directions as
shown in FIG. 6B. The vibrating wire will flex the piezoelectric
bimorph post 61 resulting in an induced emf which can be processed
as required. As the object 63 moves, the magnetic field strength at
wire 60, the vibrational amplitude of wire 60, the flexure of
bimorph post 61 and accordingly the magnitude of the induced emf
will vary thereby sensing the motion of object 63. The size of the
signal could be increased by increasing the force on wire 61.
For a given current level one way to do this would be to place
additional wires 70 on a vibrating diaphragm 71 as shown in FIGS.
7A and 7B. In that way each wire could be connected near a base 72
of diaphragm 71. In the presence of a magnet, the force on the
wires which cross in the center of diaphragm 71 would add resulting
in a large deflection of the diaphragm. This motion would be "read
out" by a piezoelectric element 73 which could form a portion of
the peripheral supporting structure at the lease of diaphragm 71.
Lead wires 75 from opposing surfaces of element 73 attach element
73 to a voltage sensing device 74. Also shown is a feedback circuit
76 discussed below which would sense the induced emf and feedback
to a power supply 77, driving coil wires 70 to maintain its
frequency always at the resonance frequency of diaphragm 71. In the
same way other vibrating structures particularly appropriate for
other applications can be designed.
Whatever the structure of the vibrating wires, as the magnet
attached to the object approaches them, the field strength may
change nonlinearly necessitating additional signal processing.
Linearity can be regained by shaping the magnet or using additional
fixed magnets. An alternate approach, shown in FIG. 8A, is to leave
the magnet 80 in a fixed position relative to a vibrating element
81 while a high magnetic permeability material 82 attached to an
object 83 intercepts the field. The shape of the permeable "shield"
(such as a wedge shown in FIG. 8B) can be designed so that the
motion of the object linearly changes the field strength at the
position of the vibrating wires thereby realizing a linear sensor
output with position. Linear response for different object motions
such as rotary travel can be accomplished with different shapes for
the shield.
In the use of resonant vibrational structures with a moderately
large Q, small dimensional variations in manufacture or changes in
material properties with temperature or pressure will cause the
resonance frequency to change. If the structure is electrically
driven somewhat off resonance, the vibrational amplitude and
accordingly the magnitude of the sensor output will decrease
rapidly as the deviation from the resonant frequency increases.
Thus, to usefully implement this concept, a convenient method is
required to electrically sense the motion of the blade and use this
signal as the input to feedback electronic circuitry which keeps
the ac current in the coil always at the resonant frequency of the
cantilever despite the fact that this resonant frequency may be
changing.
Feedback circuitry which accomplishes this objective can be
constructed by those skilled in this art. The prior art, U.S. Pat.
No. 4,297,872, discusses such techniques using phase locked loops.
In this case, the output signal from the piezoelectric element is
also the appropriate input to the feedback circuit whose output
drives the ac current through the coils. Such circuitry is
indicated by element 76 in FIG. 7A.
Electronic circuitry is also required to process the output signal
and obtain the desired information. That signal is in the form of
an amplitude modulated sine or carrier wave (at the resonant
vibrational frequency) similar to that employed in amplitude
modulated radio signals. The electronic task, familiar to those
skilled in the art, is to demodulate the signal.
An additional application of the vibrating coil-magnet assembly
would be that of a timing sensor similar to that used for engine
speed and crankshaft position sensing in current automotive
applications. The usage is illustrated in FIG. 9 where a vibrating
coil 90, magnet 93 assembly is positioned close to teeth 91 (made
of iron or other magnetic material) of a gear 92 whose rotational
motion indicates speed for example. Thus, speed would be determined
by the number of teeth passing a reference point per unit of time.
Device operation depends on the appropriate placement of magnet 93
with respect to the coil. Dashed lines 94 coming from magnet 93
suggest the form of the magnetic field.
With the tooth at its furthest distance from the coil 90, magnet 93
assembly, magnet 93 is positioned so that a substantial field is
present at coil 90 resulting in a large vibrational amplitude where
current at the resonance frequency is passed through coil 90 from
power supply 95. A large emf is present at the output of a
piezoelectric member 96. As the tooth approaches the assembly, it
captures the magnetic field decreasing its value at coil 90 and as
a result diminishes the amplitude of vibration and piezoelectric
member 96 emf. This reduction marks the passage of the tooth and
the rate of tooth passage can be determined by external signal
processing circuitry which uses the induced emf as input. The key
to the device operation is to achieve a large emf variation. This
in turn will depend on the relative sizes of magnet 93, coil 90,
teeth 91, the distance of the teeth from the coil magnet assembly,
and the magnetic properties of the material from which teeth 91 are
made.
Using cantilever blades of the sizes assumed in the prior
calculation and small cylindrical magnets (e.g., samarium cobalt
rare-earth magnets from Hitachi Magnetics Co.) of comparable
dimensions, induced emf reductions by a factor of 5 at the closest
approach of the teeth (made of iron) have been observed for the
geometry of FIG. 9 where the dimensions a and b are on the order of
2-3 mm. One advantage of this technique is that the tooth need not
come between the coil and the magnet thus allowing the same sensor
unit to be used with different toothed wheels. Secondly, this
method has the advantage that the timing signal doesn't depend on
the rotational velocity of the wheel as long as the angular
frequency of the wheel is less than the angular frequency of
vibration of the coil. The linear proportionality of pick-up signal
magnitude with rotational velocity is an important disadvantage of
some other, nonvibrational, timing sensors.
Various modifications and variations will no doubt occur to those
skilled in the art to which this invention pertains. For example,
the particular shapes and sizes of the cantilevered components and
magnets can be varied from those disclosed here. These and all
other variations which basically rely on the teachings through
which this disclosure has advanced the art are properly considered
within the scope of this invention.
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